We just migrated our forum from phpbb3 to vbulletin 3.8.3 .
All problems were solved it seems, but user conversion wasn't really successful. There are now 3500 users now instead of 2500, but some accounts are missing anyway. Some users are gone or have the guest status now, some users are moderators but weren't, some have completely different usernames, some have completely wrong number of posts. Also, simple math will tell that there are many duplicates :)
Old phpbb3 tables are still intact.
It seems like the best idea is to import users all over, somehow.